This invention relates to resistance welding and particularly to a method for joining elements of greater electrical resistance to surfaces of substantially lesser electrical resistance.
In general, methods for spot welding materials to a high quality electrical conductor such as copper or aluminum are limited by the bulk resistance and low contact resistance of the conductor. With a typical fuse-limited current supply, the maximum heat available for welding is directly proportional to the resistance placed across the welding electrodes. Copper and aluminum, being among the best conductors of electricity known, have such low resistance that extremely high current circuits were thought to be necessary to supply sufficient heat to form a junction. In addition to the special wiring required for such high current circuits, earlier inventors had to provide complex electronic means for limiting pulse voltage surges during welding. See U.S. Pat. No. 3,207,884. Alternatively, some inventors allowed the elements intended to be joined to be partly destroyed by the high current as a means for limiting current or voltage surges. See U.S. Pat. No. 3,466,419.
In the method described in U.S. Pat. No. 3,466,419, useful only for attaching conductive elements to fine resistive wire wound coils, it is necessary first to use a large current to burn a hole through the conductive element which will raise the element's resistance and then to pass a current through the fine resistive wire causing that wire to burn through, leaving its ends fused to the edges of the burn hole. That method breaks the continuity of the wire and produces a junction area limited to the tip of the wire and the rim of the hole. That method also required that the electrodes used to pass current through the conductive element be closely spaced. This spacing must be of the order of the thickness of the conductive surface and is a critical dimension in the prior art. In addition, since the electrodes carrying these high currents were themselves made of copper they would have to be sufficiently massive to act as heat sinks in order to avoid heating to a point where they would be fused to the circuit. The use of these massive electrodes was not advantageous where one of the elements to be fused was itself a massive conductive element such as a sheet of conductive metal.
Another serious restriction upon the utility of all prior art spot welding methods is that by relying on ohmic heating in a highly conductive material it was not possible to maintain the surface of the conductor relatively cool near the electrodes. Thus prior art devices tended to be characterized by burning the junction, a need to maintain critically small distances between electrodes and overheating the conductor away from the area where the junction was being formed.
In addition, although nichrome wire carrying an electric current is common in heating elements said wire commonly having the shape of a ribbon, it is presently considered unsuitable for use in conjunction with copper clad printed or semiconductor control circuits because of the difficulties of making a nichrome-copper connection that will withstand high temperatures.
At present such connections are either made as elaborate and unsatisfactory mechanically crimped connections or through the use of adhesives. This has prevented the economical use of semiconductor circuitry in connection with heating elements.
The heating industry often manufactures large duct surfaces to specifications calling for highly conductive materials such as aluminum or copper having resistivities of the order of 1-3 microhm-cm. For purposes of installation or of the attachment of insulation to these ducts it is necessary to join these duct surfaces to metal fasteners having resistivities of the order of 10-150 microhm-cm. Presently, the mass production of these duct systems is made expensive and not fully satisfactory by having to adhesively join the resistive and non-resistive metals. Welding is unsuitable for this task because the low conductivity of the duct walls would necessitate the use of unrealistically large currents attendant with all the drawbacks in the prior art just discussed.